It is noted that SDLs are also known in the art as Vertical External Cavity Emitting Lasers (VECSELs) or Optically Pumped Semiconductor Lasers (OPSLs). Therefore the term semiconductor disc laser (SDL) when used throughout the present description is used to refer to each of these systems.
The thermal sensitivity of semiconductor light-emitting devices is well known in the art, and it is also well known that devices tend to operate with greater efficiency at lower temperatures due to a decrease in carrier leakage with decreasing temperature. In an SDL structure, however, thermal effects are more complex. Optical pumping introduces a quantum defect between the pump and laser photons, producing excess heat in the gain structure which increases with increased pumping/SDL power, alongside the heating effects of non-radiative effects.
For most SDL applications, room temperature operation is desirable, indeed, the less temperature sensitive a structure is around room temperature, the better. Consequently, a great deal of skill and effort is taken within SDL systems in order to thermally manage the SDL structure.
Early techniques for thermally managing an SDL structure involved thermally mounting the SDL to a peltier controlled cooling block, as presented schematically in FIG. 1. In this arrangement the SDL 1 is bonded onto a copper mount 2 by a suitable layer of adhesive 3 e.g. conductive silver paint. The copper mount 2 is then attached to a copper block 4 by a layer of conductive heat paste 5. The temperature of the copper block 4 is then controlled by a peltier device 6 attached to a water-cooled copper heatsink 7.
When the SDL 1 shown in FIG. 1 is pumped, the heat generated must travel directly from its source within the SDL 1. The available routes for heat dissipation are primarily out through the front-surface of the SDL 1 into air, or back through the SDL 1 to the copper block 4 and eventually on to the water-cooled copper heatsink 7. The extremely low thermal conductivity (κ) of air (κ=0.026 Wm1K−1) results in a negligible amount of heat being evacuated via this route. Instead, the majority of the heat dissipates through the SDL 1 itself, which, due to its many-layer structure, and the inclusion of some low thermal conductivity layers e.g. those comprising AlGaAs (κ=22.5 Wm1K−1) and GaAs (κ=55 Wm1K−1), has a relatively high thermal impedance. It is therefore found that such a thermal management arrangement does not permit for high power lasing to be achieved.
Alternative techniques for thermally managing an SDL 1 structure are those based on crystalline heat spreaders, an example of which is schematically presented in FIG. 2. In particular, the cooling apparatus can be seen to comprise a heat spreader 8 and a standard thermoelectric or water cooler 9. The heat spreader 8 commonly employed in the art comprises a variety of materials including sapphire (κ=44 Wm1K−1), silicon carbide (κ=490 Wm1K−1) and diamond (κ=2000 Wm1K−1). The heat spreader 8 may further comprises an external, wedged face 10 with a high performance anti-reflection coating deposited thereon.
In these arrangements the heat spreader 8 is bonded by optical contacting with the SDL 1, sometimes referred to as “Van der Waals bonding”. Direct or Van der Waals bonding techniques are commonly employed within the field of semiconductor lasers as a method for joining a heat spreader 8 to an SDL 1 without the need to employ an adhesive and so avoids the associated disadvantages associated with such adhesive layers e.g. mismatches in thermal expansion coefficients that may result in cracking at high temperatures, etalon effects in the adhesion layer, impurities in the adhesive that lead to optical losses and absorption, diffusion of foreign atoms into the SDL 1 and heat-spreader structures from the adhesive, structural weaknesses in the adhesive layer itself, and thermal impedance introduced by the layer and the additional interfaces. In summary, this technique involves the polishing and cleaning of the surfaces to be bonded. A bonding liquid e.g. water, methanol or acetone is then applied to one of the surfaces to be bonded. The second surface is then brought in to contact with the first, sandwiching the bonding liquid in between. The second surface is then gently moved across the first surface until a bond is felt to form, and the sample “snatches” to the first surface.
The SDL 1 and heat spreader 8 assembly is then typically fixed on top of a layer of indium foil 11 onto the thermoelectric or water cooler 9.
The described arrangement shown in FIG. 2 allows the heat spreader 8 to immediately spread the heat generated within the SDL 1 by a pump field to the cooling apparatus 9 after it has propagated only a limited distance into the SDL 1. As a result the overall efficiency of the SDL 1 is significantly increased when compared to the previously described thermal management configurations of FIG. 1. However, thermal management techniques that incorporate optical contacting between the heat spreader 8 and the SDL 1 are found to deteriorate over time due to the effects of evaporation on the bonding liquid and the ingress of foreign bodies between the bonded layers and onto the gain medium. These effects can have a direct impact on the performance of the SDL 1 and thus are found to significantly reduce the lifetime of any SDL 1 based device.
It is therefore an object of an embodiment of the present invention to obviate or at least mitigate the foregoing disadvantages of the methods and apparatus for mounting a semiconductor disc laser known in the art.